1. Field of the Invention
The present invention relates to a method of preventing the sweating (condensation of water vapor) of the exterior panel of a freezer or refrigerator door or wall due to cold spots, particularly at the recess behind the handle where the insulation is typically thinner than the rest of the door. The typical thermal insulating material for refrigerators or freezers is blown-in polyurethane foam in the wall panel. An improved insulating material having vertical insulating properties and lateral thermal conducting properties has been found to prevent or diminish sweating. More particularly, a thermal insulating material made of alternating layers of aluminum metal foil and formed insulating foam sheets is effective in preventing sweating. This insulating material raises the temperature of a cold spot at least 3 degrees F. when compared to an equivalent amount of blown-in polyurethane foam under defined test conditions. A cold spot is an area of a surface that is at least 3 degrees F. colder than the surrounding surface.
2. Related Art
A long standing problem with freezers is condensation or sweating on the external surface of the freezer door which results from the external surface being at a temperature below the dew point temperature of the external air contacting it. The low temperature of the external surface of a door is a result of the transfer of thermal energy from the door to the interior air of the freezer. The resulting cold spot causes moisture in the external air to condense on the colder outer surfaces of the door, particularly on a recess behind the freezer handle. It also can occur on the external peripheral edges or walls of the door, particularly those proximate to gaskets.
For a number of years this problem has been addressed by applying additional heat to the areas likely to be affected by condensation. In some refrigerators and freezers this is accomplished by an elongated electric heater positioned adjacent the inside of the front face of the door. In other refrigerators a hot gas loop is positioned inside the front face of the cabinet.
This approach has a number of drawbacks. It requires substantial additional material and labor to install a heater. The source of heat is buried in the foamed insulation and is not accessible after the manufacture of the refrigerator is completed. A hot gas loop complicates the refrigeration system and provides a potential source for a system failure. Electric heaters may not have an expected life as long as that of the refrigerator. Thus manufacturers often install two heaters, with the second to be used only if the first becomes inoperable. This increases the cost of all such refrigerators when only a small percentage of the second heaters will ever be needed. This approach makes the refrigerator less energy efficient as it requires additional energy to supply the heat and additional energy for the refrigeration system to overcome the additional thermal energy that enters the compartments.
U.S. Pat. No. 3,041,679 discloses an insulated refrigerator and freezer door. The door has a shell inside the refrigerator, with an equalizer sheet of a heat conductive metal attached to an external ornamental panel. Between the shell and equalizer sheet is an air space which prevents direct contact between the shell and the equalizer sheet. This approach teaches away from the claimed invention as it is not used in conjunction with insulation that is not air, nor does this approach provide the necessary combination of vertical insulation with thermal conductivity of the present invention.
U.S. Pat. No. 5,725,294 discloses a refrigerator door with an inner panel and an outer panel forming a chamber. On the outside of the outer panel are cold spots that result in condensation. Adhered to the outer panel, in the chamber, is a sheet or strip of metal of high heat conductivity, such as aluminum or copper, which is from about 0.005 to 0.05 inches thick. The sheet or strip extends from the cold spot to adjacent areas of the door that have a temperature above the dew point temperature. The warmer adjacent areas conduct heat via the metal strip and raise the temperature of the cooler area above the dew point temperature so that condensation is eliminated. Blown in foam insulation is added behind the metal strip, filling the chamber. This cold spot insulation of '294 teaches away from the claimed invention which combines layers of insulation providing vertical insulation with layers of aluminum foil which provides thermal conductivity. Further, the '294 insulation uses thicker foil of from 0.005 to 0.05 inches (5 to 50 mil) and is designed to be used with blown in foam insulation. The present insulation is used in place of blown in foamed insulation. Finally, the use of thermal conducting material as described in '294 fails to provide the needed combination of vertical insulation and thermal conductivity provided by the present invention.
Neither of the above disclosed insulated refrigerator doors were commercially used, because neither was highly effective in preventing sweating. Neither of the above references teaches nor suggests the claimed invention.